It is often the case that various material samples need to be analyzed for (1) identification of elemental composition, molecular makeup, or mineral content, (2) study of crystallization (e.g., in the study of food shelf lives), (3) evidence of stress and shock, (4) crystallite size and orientation distributions (i.e., crystalline texture). X-ray diffraction (XRD) is one of the primary techniques used by mineralogists and solid state chemists to examine various physical and chemical properties of unknown solids.
FIG. 1 shows a schematic representation of one conventional x-ray diffraction/x-ray fluorescence (XRD/XRF) instrument designed to characterize elemental composition and mineralogy from small fine-grained or powdered samples. This XRD apparatus 32, dubbed “CHEMIN” or “CheMin” due to its ability to provide a combined CHEmical and MINeralogical analysis, is disclosed in U.S. Pat. No. 5,491,738 to Blake et al. CheMin is one embodiment of the invention described in Blake et al.
The XRD apparatus 32 of FIG. 1 was designed to analyze x-ray diffraction pattern(s) from a thin-film sample 42 disposed in a sample holder 41. The sample has a sample thickness that allows production of diffracted x-rays at the back side of the sample (e.g., a transmissive Laue method) when the thin-film sample 42 is irradiated with beam 40. An x-ray emitter 34, such as a CuKa emitter, is used to produce a broad spectrum of x-ray energies in a non-collimated beam. The non-collimated beam is passed through a collimator 38 to produce a collimated beam 40. The x-rays that are transmitted through the thin-film sample 42 are, in turn, incident upon a charge-coupled device (CCD) 46 containing a 2-dimensional planar array of pixels. The CCD 46 is an array detector adapted to record the energy of and position of individual incident X-rays.
Primary and secondary X-rays produced by irradiation of the sample 42 are directed onto the pixel array of the CCD 46. FIG. 1 shows three different diffraction cones 43, 44, and 45, which are due to diffraction from different-energy x-rays in the beam. A controller unit 48 is provided to receive input signals from each of the pixels in the CCD 46, relating to the pixel position and photon energy measured at each pixel, for use in constructing the diffraction pattern of photons within a selected energy range striking the array. A microprocessor 50 is used to provide a screen display for the controller unit and to permit control of the CCD settings by a user.
During irradiation, only a small region (e.g., 50 μm in diameter) is illuminated by the collimated X-ray beam 40. Following an exposure and data collection for a given substrate position, the thin-film sample 42 and/or x-ray emitter 34 is moved to a new position to expose another area of the thin-film sample 42. The x-rays are diffracted from the planes of atoms in the thin-film sample 42 into a spatial pattern on the CCD 46 that reveals the distribution of atoms in the sample. The spatial pattern of the diffracted X-rays detected by the CCD 46 is analyzed using Bragg's Law, which relates the wavelength (λ) of the X-ray, the atomic plane separation (d) of the sample, the diffracted angle (2θ) of the X-ray away from its original course, and the diffraction order (n) by the equation n*λ=2*d*sin(θ). For a fixed wavelength (λ), the detector must span a large enough angle (θ), so that atomic plane separations (d) can be determined.
In conventional XRD techniques, such as that described above, a sample of a material of interest is powdered and placed on a thin-film substrate, which is then disposed in a holder. The sample is typically ground to a powder (e.g., less than 10 μm to about 100 μm) using a mill or mortar and pestle or the like. In certain applications (e.g., laboratory), samples may be prepared using acetone, isopropanol, pentane, or the like to form a uniform slurry, so as to minimize any potential problem with preferred orientation which may accompany rod-like or plate-like crystals. The prepared sample is then positioned within the XRD instrument and illuminated with x-rays, typically of a fixed wavelength, and the intensity of the diffracted radiation is recorded. The sample and/or x-ray source is then repositioned. Ideally, a large number of crystallites in random orientations are exposed to the X-ray beam, which is typically done by moving the specimen in the beam to analyze a larger number of crystallites and/or larger number of orientations of crystallites. This data is then analyzed for the diffraction angle to calculate the inter-atomic spacing (D) and the intensity (I) is measured to discriminate (using I ratios) the various D spacings and the results are used to identify possible matches when compared to known values (e.g., “The International Tables for Crystallography”, the “International Center for Diffraction Data®” (ICDD) Powder Diffraction File™ covering over 550,000 compounds, etc.).
In still other conventional techniques, in single-crystal XRD, a goniometer is used to rotate a single crystal so that many facets and sets of atomic planes are oriented so as to diffract monochromatic rays onto a fixed detector. A pattern of diffraction spots (“Laue spots”) results which, through traditional crystallographic algorithms, may be inverted to determine the underlying geometric arrangement of the atoms in the crystal.
However, conventional XRD/XRF instruments require either significant sample preparation and/or require the ability to reposition the instrument through a wide range of angles around the sample, or rotate the sample through a wide range of angles, so as to get desired sample information (e.g., identification of elemental compositions, identification of molecular makeup, identification of mineral content, study of crystallization, evidence of stress and/or shock in the material, crystal grain size, crystal orientation distributions, etc.). In either case, several moving parts are required to perform the XRD analysis. However, in certain applications, such as extraterrestrial XRD analysis, the number of moving parts required increases (e.g., the CheMin device uses a carousel disc and associated drive system, sample preparation systems such as a fine-grinding mill, etc.), with corresponding increases in power consumption, mass, and risk. For the CheMin XRD/XRF apparatus 32, which is presently slated for inclusion on the Mars Science Laboratory (MSL) mission scheduled for launch in 2009, sample preparation is required, which would disadvantageously destroy any water ice that MSL may encounter and cause it to evaporate in the low pressure environment on Mars. Sample preparation also destroys valuable scientific and engineering information regarding grain size and orientation distributions and evidence of stresses and shock.